U.S. patent application number 14/305267 was filed with the patent office on 2014-11-06 for zinc oxide sputtering target and method for producing same.
The applicant listed for this patent is NGK INSULATORS, LTD.. Invention is credited to Katsuhiro IMAI, Koichi KONDO, Jun YOSHIKAWA.
Application Number | 20140328747 14/305267 |
Document ID | / |
Family ID | 48905009 |
Filed Date | 2014-11-06 |
United States Patent
Application |
20140328747 |
Kind Code |
A1 |
YOSHIKAWA; Jun ; et
al. |
November 6, 2014 |
ZINC OXIDE SPUTTERING TARGET AND METHOD FOR PRODUCING SAME
Abstract
Provided is a zinc oxide sputtering target, which can
effectively suppress the occurrence of break or crack in the target
during sputtering to enable production of a zinc oxide transparent
conductive film with high productivity. The zinc oxide sputtering
target is composed of a zinc oxide sintered body comprising zinc
oxide crystal grains, wherein the zinc oxide sputtering target has
a sputter surface having a (100) crystal orientation degree of 50%
or more.
Inventors: |
YOSHIKAWA; Jun; (Nagoya-Shi,
JP) ; IMAI; Katsuhiro; (Nagoya-Shi, JP) ;
KONDO; Koichi; (Nagoya-Shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGK INSULATORS, LTD. |
Nagoya-shi |
|
JP |
|
|
Family ID: |
48905009 |
Appl. No.: |
14/305267 |
Filed: |
June 16, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2013/050905 |
Jan 18, 2013 |
|
|
|
14305267 |
|
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Current U.S.
Class: |
423/622 ;
204/298.13; 264/605; 264/638; 264/650; 264/681 |
Current CPC
Class: |
C04B 2237/34 20130101;
C04B 2235/6025 20130101; C04B 35/453 20130101; C04B 35/64 20130101;
C04B 2235/5445 20130101; C04B 2235/786 20130101; C04B 2235/788
20130101; C23C 14/086 20130101; C04B 2235/549 20130101; C04B
2235/787 20130101; H01J 2237/081 20130101; C04B 2235/9607 20130101;
B32B 18/00 20130101; C04B 2237/407 20130101; C04B 35/638 20130101;
C04B 35/634 20130101; C04B 35/6261 20130101; C01G 9/02 20130101;
C04B 2235/5292 20130101; C04B 2237/12 20130101; C04B 37/026
20130101; H01J 37/3426 20130101; C04B 2111/0037 20130101; C23C
14/3414 20130101 |
Class at
Publication: |
423/622 ;
204/298.13; 264/605; 264/638; 264/650; 264/681 |
International
Class: |
H01J 37/34 20060101
H01J037/34; C04B 35/64 20060101 C04B035/64; C01G 9/02 20060101
C01G009/02; C04B 35/453 20060101 C04B035/453 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2012 |
JP |
2012-016456 |
Sep 25, 2012 |
JP |
2012-211222 |
Claims
1. A zinc oxide sputtering target composed of a zinc oxide sintered
body comprising zinc oxide crystal grains, wherein the zinc oxide
sputtering target has a sputter surface having a (100) crystal
orientation degree of 50% or more.
2. The zinc oxide sputtering target according to claim 1, wherein
the orientation degree is 75% or more.
3. The zinc oxide sputtering target according to claim 1, wherein
the crystal grains have an aspect ratio of 2.0 or lower in a cross
section perpendicular to the sputter surface.
4. The zinc oxide sputtering target according to claim 3, wherein
the aspect ratio is 1.5 or lower.
5. The zinc oxide sputtering target according to claim 1, wherein
the zinc oxide crystal grains have an average grain diameter of 1
to 50 .mu.m.
6. A method for producing a zinc oxide sputtering target comprising
the steps of: providing a plate-like zinc oxide powder having a
mean volume particle diameter D50 of 0.1 to 1.0 .mu.m; orienting
the plate-like zinc oxide powder through a method utilizing
shearing stress to obtain an crystallographically oriented green
compact; and firing the crystallographically oriented green compact
at a firing temperature of 1000 to 1400.degree. C. to obtain a zinc
oxide sintered body comprising zinc oxide crystal grains that are
oriented.
7. The method according to claim 6, wherein the method utilizing
shearing stress is at least one of selected from the group
consisting of a tape casting, an extrusion molding, and a doctor
blade method.
8. The method according to claim 6, wherein the method utilizing
shearing stress is carried out by preparing a slurry from the
plate-like zinc oxide powder and passing the slurry through a
discharge port to obtain the crystallographically oriented green
compact in a sheet-like form.
9. The method according to claim 8 further comprising, prior to the
firing, the steps of preparing a plurality of the sheet-like
crystallographically oriented green compacts; stacking the
sheet-like crystallographically oriented green compacts on top of
each other to provide a precursor laminate; and subjecting the
laminate precursor to a press molding.
10. The method according to claim 6, wherein the step of providing
the plate-like zinc oxide powder comprises adding an alkali aqueous
solution to a zinc salt aqueous solution to provide a mixture;
stirring the mixture at a temperature of 60 to 95.degree. C. for 2
to 10 hours to form a precipitate; and subjecting the precipitate
to washing, drying and pulverization.
11. The method according to claim 10, wherein the pulverization is
performed for 1 to 10 hours by using a ball mill.
12. A zinc oxide transparent conductive film obtained by sputtering
using the sputtering target according to claim 1.
13. A zinc oxide transparent conductive film obtained by sputtering
using the sputtering target produced by the process according to
claim 6.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of
PCT/JP2013/050905 filed Jan. 18, 2013, which claims priorities to
Japanese Patent Application No. 2012-16456 filed Jan. 30, 2012 and
Japanese Patent Application No. 2012-211222 filed Sep. 25, 2012,
the entire contents of all of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a zinc oxide sputtering
target and a method for producing the same.
[0004] 2. Description of the Related Art
[0005] For a transparent conductive film used in electronic devices
and the like, indium tin oxide (ITO) and the like have been widely
used over many years. However, in view of the situation where the
prices of rare earth metals such as indium have soared in recent
years, rare metal alternatives are strongly desired. Recent
vigorous attempts to prepare a transparent conductive film with
less expensive zinc oxide (ZnO) have not genuinely come into
practical use as having various problems such as failure to attain
a desired conductivity due to difficulty in sufficiently lowering
electrical resistivity.
[0006] Meanwhile, it is the industrial mainstream that a
transparent conductive film is produced by sputtering. A sputtering
target reaches a high temperature during sputtering, hence the
sputtering target is cooled by a backing plate having water
channels inside. Thus, a temperature difference between the sputter
surface exposed to a high temperature and the water-cooled backing
plate surface tends to generate heat stress inside the sputtering
target, which may cause cracks in the sputtering target.
[0007] Patent Document 1 (JP2009-215629A) discloses that generation
of cracks can be suppressed if a coefficient of linear thermal
expansion in the direction perpendicular to the sputter surface of
the target is more than 10% greater than that in the direction
parallel the sputter surface. However, the effect of suppressing
the occurrence of cracks was not sufficient when the power supply
was increased for achieving higher productivity during
sputtering.
[0008] Patent Document 2 (JP3128861B) and Patent Document 3
(JP3301755B) disclose a sputtering target composed of a zinc oxide
sintered body. Although these documents disclose that the quality
of the sputtered film can be improved by having zinc oxide oriented
along with the (101) or (002) plane, such improvement was not
intended to suppress the occurrence of cracks in the target.
CITATION LIST
Patent Documents
[0009] Patent Document 1: JP 2009-215629A
[0010] Patent Document 2: JP 3128861B
[0011] Patent Document 3: JP 3301755B
SUMMARY OF THE INVENTION
[0012] The inventors have currently found that allowing a zinc
oxide sputtering target to have a sputter surface with a (100)
crystal orientation degree of 50% or more makes it possible to
effectively suppress the occurrence of break or crack in the target
during sputtering, and thus enables production of a zinc oxide
transparent conductive film with high productivity.
[0013] Thus, an object of the present invention is to provide a
zinc oxide sputtering target, which can effectively suppress the
occurrence of break or crack in the target during sputtering to
enable production of a zinc oxide transparent conductive film with
high productivity.
[0014] According to an aspect of the present invention, there is
provided a zinc oxide sputtering target composed of a zinc oxide
sintered body comprising zinc oxide crystal grains, wherein the
zinc oxide sputtering target has a sputter surface having a (100)
crystal orientation degree of 50% or more.
[0015] According to another aspect of the present invention, there
is provided a method for producing a zinc oxide sputtering target
comprising the steps of: [0016] providing a plate-like zinc oxide
powder having a mean volume particle diameter D50 of 0.1 to 1.0
.mu.m; [0017] orienting the plate-like zinc oxide powder through a
method utilizing shearing stress to obtain a crystallographically
oriented green compact; and [0018] firing the crystallographically
oriented green compact at a firing temperature of 1000 to
1400.degree. C. to obtain a zinc oxide sintered body comprising
zinc oxide crystal grains that are oriented.
[0019] According to another aspect of the present invention, there
is provided is a zinc oxide transparent conductive film obtained by
sputtering using the sputtering target according to any one of the
above aspects.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIG. 1 is an SEM image of spherical secondary particles
containing aggregated plate-like zinc oxide primary particles
prepared in Example 1.
[0021] FIG. 2 is an SEM image of plate-like primary particles after
ball mill crushing prepared in Example 1.
[0022] FIG. 3 is an SEM image of a cross section of a zinc oxide
sintered body after polishing and etching taken in Example 2.
[0023] FIG. 4 is an XRD profile of a zinc oxide sintered body
measured in Example 2.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Zinc Oxide Sputtering Target
[0025] The zinc oxide sputtering target of the present invention is
composed of a zinc oxide sintered body comprising zinc oxide
crystal grains, and has a sputter surface having a (100) crystal
orientation degree of 50% or more, preferably 75% or more, and more
preferably 90% or more. In this way, orienting the (100) crystal
plane of zinc oxide parallel to the sputter surface can effectively
suppress the occurrence of break or crack in the target during
sputtering to enable production of a zinc oxide transparent
conductive film with high productivity.
[0026] Specifically, zinc oxide crystal has a hexagonal wurtzite
structure, of which the coefficient of thermal expansion in the
c-axis direction is 4.5.times.10.sup..times.61.degree. C., which is
lower than 7.8.times.10.sup.-6/.degree. C. in the a-axis direction.
Since the (100) crystal plane of zinc oxide is a plane that is
parallel to the c-axis direction of the crystal, increasing the
(100) crystal orientation degree in the sputter surface to as high
as 50% or more allows the c-axis, which has a coefficient of
thermal expansion that is lower than that of the a-axis, to be
oriented parallel to the in-plane direction of the sputter surface.
On the other hand, during sputtering, a temperature difference
between the sputter surface exposed to a high temperature and the
water-cooled backing plate surface tends to generate heat stress
inside the sputtering target, and this heat stress also occurs in
the in-plane direction of the sputter surface. This is because the
sputtered side of the target leads to a large thermal expansion due
to a high temperature, while the other side of the target facing
the backing plate leads to a small thermal expansion due to a low
temperature, and such difference generates distortion in the
in-plane direction of the sputter surface. No such heat stress
occurs in the thickness direction of the target. Thus, orienting
the (100) plane of the zinc oxide crystal parallel to the sputter
surface allows the c-axis, which has a coefficient of thermal
expansion that is lower than that of the a-axis, to be oriented
parallel to the in-plane direction of the sputter surface, along
which heat stress occurs, and thus to reduce the thermal expansion
in the in-plane direction of the sputter surface, which may be a
cause of break or crack.
[0027] The zinc oxide sintered body comprises zinc oxide crystal
grains. That is, the zinc oxide sintered body is a solid object in
which a plurality of zinc oxide crystal grains bond together by
sintering. The zinc oxide crystal grains are grains comprising zinc
oxide, and may also comprise, as other elements, a dopant such as
Al and Ga belonging to the Group 3B elements and the like, and
inevitable impurities, or may be composed of zinc oxide and
inevitable impurities. Such other elements may be substituted for
Zn or O site of the hexagonal wurtzite structure, may be included
as an additive element which does not constitute the crystal
structure, or may exist at a grain boundary. The zinc oxide
sintered body may also comprise another phase or another element as
described above in addition to zinc oxide crystal grains, but is
preferably composed of zinc oxide crystal grains and inevitable
impurities.
[0028] The zinc oxide sintered body of the present invention has a
sputter surface having a (100) crystal orientation degree of 50% or
more, preferably 75% or more, and more preferably 90% or more. The
higher degree of the (100) crystal orientation in the sputter
surface can more effectively reduce heat stress in the in-plane
direction of the target, which may be a cause of break or crack.
Therefore, an upper value of degree of the (100) crystal
orientation in the sputter surface should not be particularly
limited and is ideally 100%. The value of the (100) crystal
orientation degree can be determined by using an XRD equipment
(e.g., product name "RINT-TTR III" manufactured by Rigaku
Corporation) to measure an XRD profile upon X-ray irradiation on a
surface of a zinc oxide sintered body in a disk-shaped form. The
value of F representing the (100) crystal orientation degree is
calculated by the following formulae.
F = p - p 0 1 - p 0 p 0 = I 0 ( 100 ) I 0 ( 100 ) + I 0 ( 002 ) + I
0 ( 101 ) + I 0 ( 102 ) p = I s ( 100 ) I s ( 100 ) + I s ( 002 ) +
I s ( 101 ) + I s ( 102 ) [ Formula 1 ] ##EQU00001##
(I.sub.0(hkl) and I.sub.s(hkl) respectively represent diffraction
intensities from (hkl) planes (integrated value) in ICDD No. 361451
and a sample.
[0029] The zinc oxide crystal grains preferably have an aspect
ratio of 2.0 or lower, more preferably 1.5 or lower, and further
preferably 1.3 or lower in a cross section perpendicular to the
sputter surface. This aspect ratio is a ratio of (a length parallel
to the sputter surface)/(a length perpendicular to the sputter
surface). With this ratio being closer to 1, the anisotropy is more
reduced to come closer to the isotropy and thus makes delamination
upon orientation less likely to occur between crystal faces so as
to enhance the strength, thereby contributing to suppression of
break or crack. This aspect ratio can be determined in the
following manner. That is, a cubic-shaped sample of which the one
side is 5 mm is cut out from a zinc oxide sintered body in a
disk-shaped form (a sputtering target). After polishing a surface
perpendicular to the disk plane and etching it with 0.3 M nitric
acid for 10 seconds, an image of the sample is taken by a scanning
electron microscope. The visual field range is chosen in such a
manner that any straight lines, when drawn parallel and
perpendicular to the disk plane, intersect 10 to 30 grains. In 3
straight lines drawn parallel to the disk plane, an average length
of line segment passing through each grain is calculated in all
grains that the straight lines intersect, and then is multiplied by
1.5 to give a value of a.sub.1. Similarly, in 3 straight lines
drawn perpendicular to the surface of the disk plane, an average
length of line segment passing through each grain is calculated in
all grains that the straight lines intersect, and then is
multiplied by 1.5 to give a value of a.sub.2. The aspect ratio is
given as a value of a.sub.1/a.sub.2.
[0030] The zinc oxide crystal grains preferably have an average
grain diameter of 1 to 50 .mu.m, more preferably 2 to 35 .mu.m,
further preferably 3 to 20 .mu.m, and most preferably 3 to 10
.mu.m. Having such an average grain diameter sufficiently reduces
the aspect ratio of the zinc oxide crystal grains and increases the
strength of the sintered body, making it possible to suppress break
or crack more effectively. The average grain diameter can be
determined in the following manner. That is, a cubic-shaped sample
of which the one side is 5 mm is cut out from a zinc oxide sintered
body in a disk-shaped form (a sputtering target). After polishing a
surface perpendicular to the disk plane and etching it with 3M
nitric acid for 10 seconds, an image of the sample is taken by a
scanning electron microscope. The visual field range is chosen in
such a manner that any straight lines, when drawn parallel and
perpendicular to the disk plane, intersect 10 to 30 grains. In 3
straight lines drawn parallel to the surface of the disk plane, an
average length of line segment passing through each grain is
calculated in all grains that the straight lines intersect, and
then be multiplied by 1.5 to give a value of a.sub.1. Similarly, in
3 straight lines drawn perpendicular to the disk plane, an average
length of line segment passing through each grain is calculated in
all grains that the straight lines intersect, and then be
multiplied by 1.5 to give a value of a.sub.2. The average grain
diameter is given as a value of (a.sub.1+a.sub.2)/2.
[0031] Production Method
[0032] The zinc oxide sputtering target of the present invention as
described above may be produced in the following manner.
[0033] The first step is to provide a plate-like zinc oxide powder
having a mean volume particle diameter D50 of 0.1 to 1.0 .mu.m,
preferably 0.3 to 0.8 .mu.m. The mean volume particle diameter D50
can be measured by a laser diffraction particle size analyzer. As
such, the production method of the present invention is
characterized by using a plate-like raw powder having fine
particles at the sub-micron level.
[0034] Conventionally, an oriented sintered body was produced by
using a plate-like raw powder with a diameter of several .mu.m and
orienting the plate-like particles through a press molding or the
like (e.g., see Patent Document 1). However, the oriented sintered
body produced in such method usually has a high aspect ratio in
crystal grains, which in turn reduces the in-plane strength and
tends to generate break or crack due to heat stress. That is,
according to the findings of the present inventors, if the
plate-like coarse raw powder with a diameter of several .mu.m is
used, the aspect ratio in crystal grains of the sintered body
hardly changes over grain growth and the degree of anisotropy tends
to remain high. In contrast, in the present invention, by using the
plate-like raw powder having fine particles of the sub-micron level
as described above and employing an orientation technique utilizing
shearing stress, it is possible to obtain a sintered body with high
orientation degree even from a fine raw material. Furthermore, by
allowing the plate-like raw powder having fine particles of the
sub-micron level to undergo grain growth during sintering
(preferably to 3 .mu.m or more), the aspect ratio of the crystal
grains in the sintered body becomes low, the degree of anisotropy
is significantly reduced, and break or crack is effectively
suppressed.
[0035] Such plate-like raw powder having fine particles of the
sub-micron level may be produced through any method, but can be
preferably obtained by adding an alkali aqueous solution to a zinc
salt aqueous solution, stirring the mixture at a temperature of 60
to 95.degree. C. for 2 to 10 hours to form a precipitate, and
subjecting the precipitate to washing, drying and pulverization.
The zinc salt aqueous solution may be any aqueous solution
containing a zinc ion, but is preferably an aqueous solution of
zinc salt such as zinc nitrate, zinc chloride, and zinc acetate.
The alkali aqueous solution is preferably an aqueous solution of
sodium hydroxide, potassium hydroxide, or the like. Although the
concentrations of the zinc salt aqueous solution and the alkali
aqueous solution and the mixture ratio thereof are not particularly
limited, it is preferable to mix the same molar concentrations of
the zinc salt aqueous solution and the alkali aqueous solution in
the same volume ratio. It is preferable to wash the precipitate
with ion-exchanged water a plurality of times. The alkali aqueous
solution is more preferred to be an aqueous solution of a
nitrogen-containing basic organic compound such as tetramethyl
ammonium hydroxide, tetraethyl ammonium hydroxide, and guanidine
since it becomes possible to suppress contamination with an element
of the alkali metal group or an element of the alkaline earth
group. The washed precipitate is dried preferably at a temperature
of 100 to 300.degree. C. The dried precipitate is in the form of
spherical secondary particles containing aggregated plate-like zinc
oxide primary particles, and thus is preferred to be subjected to a
pulverization process. The pulverization is preferably carried out
in a ball mill for 1 to 10 hours with a solvent such as ethanol
added to the washed precipitate. The pulverization provides a
plate-like zinc oxide powder as primary particles.
[0036] The plate-like zinc oxide powder is oriented through a
technique utilizing shearing stress to form a crystallographically
oriented green compact. During this process, the plate-like zinc
oxide powder may be added with another element or constituent such
as a metal oxide powder (e.g., an .alpha.-Al.sub.2O.sub.3 powder)
as a dopant. Preferable examples of the technique utilizing
shearing stress include a tape casting, an extrusion molding, a
doctor blade method, and any combinations thereof. The orientation
technique utilizing shearing stress, in any method as described
above, is preferred to be conducted by preparing a slurry from the
plate-like zinc oxide powder with suitable addition of an additive
such as a binder, a plasticizer, a dispersant and a dispersion
medium, and passing the resulting slurry through a thin slit-like
discharge port so as to be discharged and molded in a sheet-like
form on a substrate. The width of the slit-like discharge port is
preferably 10 to 400 .mu.m. The amount of the dispersion medium in
use is preferably adjusted to bring the slurry viscosity to 5000 to
100000 cP, more preferably 20000 to 60000 cP. The thickness of the
crystallographically oriented green compact in a sheet-like form is
preferably 5 to 500 .mu.m, more preferably 10 to 200 .mu.m. It is
preferable that a plurality of the crystallographically oriented
green compacts in a sheet-like form are stacked on top of each
other to provide a precursor laminate having a desired thickness,
which is subjected to a press molding. This press molding can be
preferably carried out by packing the precursor laminate in a way
such as vacuum pack and subjecting the packed precursor laminate to
an isostatic pressing in hot water of 50 to 95.degree. C. at a
pressure of 10 to 2000 kgf/cm.sup.2. In the case of using an
extrusion molding, sheet-like green compacts, which have passed
through a thin discharge port in a mold, may be unified in the mold
so as to be discharged in a laminated form, in accordance with the
design of a flow path in the mold. The green compacts thus obtained
are preferably degreased according to the well-known
conditions.
[0037] The crystallographically oriented green compacts obtained as
described above are fired at a firing temperature of 1000 to
1400.degree. C., preferably 1100 to 1350.degree. C. to form a zinc
oxide sintered body comprising zinc oxide crystal grains that are
oriented. The firing time period at the above firing temperature is
not particularly limited but is preferably 1 to 10 hours, and more
preferably 2 to 5 hours. The zinc oxide sintered body thus obtained
has a high (100) crystal orientation degree in the sputter surface,
preferably 50% or more, and typically has crystal grains with an
aspect ratio of 2.0 or lower in a cross section perpendicular to
the sputter surface.
EXAMPLES
Example 1
[0038] A zinc oxide powder as a raw material was prepared in the
following manner. Zinc nitrate hexahydrate (manufactured by Kanto
Chemical Co., Inc.) was used to prepare an aqueous solution of 0.1
M Zn(NO.sub.3).sub.2. Also, sodium hydroxide (manufactured by
Sigma-Aldrich Co., Inc.) was used to prepare an aqueous solution of
0.1 M NaOH. The Zn(NO.sub.3).sub.2 aqueous solution was added to
the NaOH aqueous solution in the ratio of 1:1 by volume, and the
resulting mixture was stirred at a temperature of 80.degree. C. for
6 hours to yield a precipitate. The precipitate was washed three
times with ion-exchanged water and dried to obtain spherical
secondary particles containing aggregated plate-like zinc oxide
primary particles. FIG. 1 shows an image of the resulting secondary
particles taken by an electron microscope. Subsequently, ZrO.sub.2
balls with a diameter of 2 mm were used with ethanol as a solvent
to perform ball milling pulverization for 3 hours so as to
pulverize the zinc oxide secondary particles shown in FIG. 1 into
plate-like primary particles having a mean volume particle diameter
D50 of 0.6 .mu.m. FIG. 2 shows an image of the resulting plate-like
primary particles taken by an electron microscope.
[0039] The resulting plate-like zinc oxide primary particles in an
amount of 100 parts by weight were mixed with a binder (polyvinyl
butyral: product No. BM-2, manufactured by Sekisui Chemical Co.,
Ltd.) in an amount of 15 parts by weight, a plasticizer (DOP:
di-(2-ethylhexyl)phthalate, manufactured by Kurogane Kasei Co.,
Ltd.) in an amount of 6.2 parts by weight, a dispersant (product
name: Rheodol SP-030, manufactured by Kao Corporation) in an amount
of 3 parts by weight, and a dispersion medium (2-ethylhexanol). The
amount of the dispersion medium in use was adjusted to bring the
slurry viscosity to 10000 cP. The slurry thus obtained was
subjected to a doctor blade method to form on a PET film a sheet
having a thickness of 20 .mu.m after dried. The resulting tape was
cut into a circle with a diameter of 140 mm and 500 pieces of the
cut tapes were stacked on top of each other, and then placed on an
aluminum plate with a thickness of 10 mm to be sealed into a vacuum
pack. The vacuum pack was subjected to isostatic pressing in hot
water of 85.degree. C. at a pressure of 100 kgf/cm.sup.2 to provide
a disk-shaped green compact. The disk-shaped green compact was
placed in a degreasing furnace, in which degreasing was conducted
at a temperature of 600.degree. C. for 20 hours. The degreased body
thus obtained was fired under a normal pressure at a temperature of
1300.degree. C. for 5 hours in the air to obtain a zinc oxide
sintered body in a disk-shaped form as a sputtering target.
[0040] The value of the (100) crystal orientation degree of the
resulting zinc oxide sintered body was measured by an XRD. This
measurement was performed by using an XRD equipment (product name
"RINT-TTR III" manufactured by Rigaku Corporation) to measure an
XRD profile upon X-ray irradiation on the surface of the zinc oxide
sintered body in a disk-shaped form. The value of F representing
the (100) crystal orientation degree is calculated by the following
formulae. The value of F in the present Example was 0.55.
F = p - p 0 1 - p 0 p 0 = I 0 ( 100 ) I 0 ( 100 ) + I 0 ( 002 ) + I
0 ( 101 ) + I 0 ( 102 ) p = I s ( 100 ) I s ( 100 ) + I s ( 002 ) +
I s ( 101 ) + I s ( 102 ) [ Formula 2 ] ##EQU00002##
(I.sub.0(hkl) and I.sub.s(hkl) respectively represent diffraction
intensities from (hkl) planes (integrated value) in ICDD No. 361451
and a sample.
[0041] A cubic-shaped sample of which the one side was 5 mm was cut
out from the disk-shaped zinc oxide sintered body to measure the
average coefficient of thermal expansion in the temperature range
from 25 to 1000.degree. C. The coefficient of thermal expansion in
the in-plane direction was 6.3.times.10.sup.-6/.degree. C., while
the coefficient of thermal expansion in the thickness direction was
7.2.times.10.sup.-6/.degree. C.
[0042] The average grain diameter and the aspect ratio of the
sintered body grains were measured in the following manner. In a
sample having the same shape as that for measuring the coefficient
of thermal expansion, after polishing a surface perpendicular to
the disk plane and etching it with 0.3 M nitric acid for 10
seconds, an image of the sample was taken by a scanning electron
microscope. The visual field range was chosen in such a manner that
any straight lines, when drawn parallel and perpendicular to the
disk plane, intersected 10 to 30 grains. In 3 straight lines drawn
parallel to the disk plane, an average length of line segment
passing through each grain was calculated in all grains that the
straight lines intersected, and then was multiplied by 1.5 to give
a value of a.sub.1. Similarly, in 3 straight lines drawn
perpendicular to the disk plane, the average length of line segment
passing through each grain was calculated in all grains that the
straight lines intersected, and then was multiplied by 1.5 to give
a value of a.sub.2. The aspect ratio and the average grain diameter
were given as values of a.sub.1/a.sub.2 and (a.sub.1+a.sub.2)/2,
respectively.
[0043] Another sintered body in a disk-shaped form (a sputtering
target) produced in the same condition was bonded to a backing
plate made of copper by means of indium, and placed in an RF
magnetron sputtering equipment. In this RF magnetron sputtering
equipment, sputtering was carried out for 30 minutes in pure Ar
atmosphere at the pressure of 0.5 Pa with an input power of 150 W.
As a result of performing sputtering on 5 targets, no break was
found in any target.
Example 2
[0044] Production of a target and sputtering thereon were carried
out in the same manner as in Example 1, except that the amount of
the dispersion medium in use was reduced to bring the slurry
viscosity to 45000 cP. As a result, the orientation degree was
increased to 0.97. An SEM image of a cross section of the zinc
oxide sintered body after polishing and etching is shown in FIG. 3.
As shown in FIG. 3, it can be understood that the plate-like zinc
oxide particles of the raw powder grew to provide an isotropic
crystal grain structure. An XRD profile of the zinc oxide sintered
body is shown in FIG. 4. As shown in FIG. 4, the peak of
diffraction intensity from the (100) crystal plane is significantly
high, and thus it can be understood that the (100) crystal plane is
highly oriented. There was no break in any of the 5 targets.
[0045] Example 3
[0046] Production of a target and sputtering thereon were carried
out in the same manner as in Example 2, except that the firing
temperature was 1200.degree. C. There was no break in any
target.
[0047] Example 4
[0048] Production of a target and sputtering thereon were carried
out in the same manner as in Example 2, except that the firing
temperature was 1350.degree. C. There was no break in any
target.
[0049] Example 5
[0050] To the composition described in Example 1,
.alpha.-Al.sub.2O.sub.3 having an average particle diameter of 0.4
.mu.m in an amount of 2 parts by weight was added. The other
conditions in production were the same as in Example 2. There was
no break in any target. In the measurement of the particle
diameter, microparticles (those having an inscribed circle diameter
of 500 nm or less) presumed to be ZnAl.sub.2O.sub.4 phase was
excluded.
Example 6 (Comparative)
[0051] A commercially available zinc oxide powder (manufactured by
Seido Chemical Industry Co., Ltd zinc oxide of JIS 1 grade, mean
volume particle diameter D50 of 0.6 .mu.m) was subjected to
uniaxial pressing to produce a disk-shaped green compact, followed
by isostatic pressing at a pressure of 2 tf/cm.sup.2. After fired
at a temperature of 1300.degree. C. for 5 hours, a sintered body
having the orientation degree of 0.02 was obtained. The sputtering
was carried out in the same manner as in Example 1. As a result,
break was found in all targets.
Example 7
[0052] The slurry prepared in Example 1 was used to prepare a
sheet-like green compact having a thickness of 4 .mu.m. The
sheet-like green compact, without having been stacked in the form
of layers, was degreased at a temperature of 650.degree. C. for 20
hours and then fired at a temperature of 1300.degree. C. for 5
hours to provide a zinc oxide sintered body in the sheet-like form.
The resulting zinc oxide sintered body in the sheet-like form was
coarsely crushed in a mortar, and subsequently pulverized in a ball
mill to a mean volume particle diameter D50 of 20 .mu.m so as to
provide a plate-like zinc oxide powder. The plate-like zinc oxide
powder thus obtained and the commercially available zinc oxide
powder used in Example 6 were mixed in the ratio of 1:1 by volume,
and the mixture was subjected to uniaxial pressing at a pressure of
200 kgf/cm .sup.2 to produce a pressed green compact. The pressed
green compact was fired at a temperature of 1350.degree. C. for 5
hours to provide a sintered body. The resulting sintered body was
used as a target to carry out sputtering in the same manner as in
Example 1. As a result, break was found in one target among 5
targets.
[0053] The results obtained in Examples 1 to 7 are shown in Table
1.
TABLE-US-00001 TABLE 1 Coefficient Coefficient of thermal Break in
of thermal expansion target expansion in Average (number of in
in-plane thickness Grain grain incident/ (100) direction direction
aspect diameter total Example orientation (1/.degree. C.)
(1/.degree. C.) ratio (.mu.m) 5 targets) 1 0.55 6.3 .times.
10.sup.-6 7.2 .times. 10.sup.-6 1.1 12 0 2 0.97 6.1 .times.
10.sup.-6 7.6 .times. 10.sup.-6 1.1 14 0 3 0.78 6.2 .times.
10.sup.-6 7.3 .times. 10.sup.-6 1.3 3.5 0 4 0.95 6.1 .times.
10.sup.-6 7.6 .times. 10.sup.-6 1.0 35 0 5 0.92 6.3 .times.
10.sup.-6 7.6 .times. 10.sup.-6 1.1 5 0 6 0.02 6.6 .times.
10.sup.-6 6.6 .times. 10.sup.-6 1.0 12 5 7 0.80 6.1 .times.
10.sup.-6 7.4 .times. 10.sup.-6 3.5 41 1
* * * * *